What is a penumbral eclipse of the moon?

A penumbral eclipse is more subtle, and more difficult to observe, than a total or partial eclipse. It happens when the moon moves through the outer part of Earth’s shadow.

An eclipse of the moon can only happen at full moon, when the sun, Earth and moon line up in space, with Earth in the middle. At such times, Earth’s shadow falls on the moon, creating a lunar eclipse. When this happens – and it happens two to four times every year – everyone on Earth’s night side can see the eclipse. There are three kinds of lunar eclipses: total, partial and penumbral.

In a total eclipse of the moon, the inner part of Earth’s shadow, called the umbra, falls on the moon’s face. At mid-eclipse, the entire moon is in shadow, which may appear blood red.

In a partial lunar eclipse, the umbra takes a bite out of only a fraction of the moon. The dark bite grows larger, and then recedes, never reaching the total phase.

In a penumbral lunar eclipse, only the more diffuse outer shadow of Earth falls on the moon’s face. This third kind of lunar eclipse is much more subtle, and much more difficult to observe, than either a total or partial eclipse of the moon. There is never a dark bite taken out of the moon, as in a partial eclipse. The eclipse never progresses to reach the dramatic minutes of totality. At best, at mid-eclipse, very observant people will notice a dark shading on the moon’s face. Others will look and notice nothing at all.

View larger. | Left, an ordinary full moon with no eclipse. Right, full moon in penumbral eclipse on November 20, 2002. Master eclipse photographer Fred Espenak took this photo when the moon was 88.9% immersed in Earth’s penumbral shadow. There’s no dark bite taken out of the moon. A penumbral eclipse creates only a dark shading on the moon’s face.

In a lunar eclipse, Earth’s shadow falls on the moon. If the moon passes through the dark central shadow of Earth – the umbra – a partial or total lunar eclipse takes place. If the moon only passes through the outer part of the shadow (the penumbra), a subtle penumbral eclipse occurs. Diagram via Fred Espenak’s Lunar Eclipses for Beginners.

According to eclipse expert Fred Espenak, about 35% of all eclipses are penumbral. Another 30% are partial eclipses, where it appears as if a dark bite has been taken out of the moon. And the final 35% go all the way to becoming total eclipses of the moon, a glorious event.

Bottom line: There are three kinds of lunar eclipses: total, partial and penumbral. A penumbral eclipse is very subtle. At no time does a dark bite appear to be taken out of the moon. Instead, at mid-eclipse, observant people will notice a shading on the moon’s face. They might say the moon looks strange. Others will notice nothing at all.

Here’s what a partial eclipse looks like. This is the partial eclipse June 4, 2012. Astronomer Alan Dyer took this photo from his home in southern Alberta, Canada. It was pre-dawn, near moonset. See more of Dyer’s photos here. Image copyright Alan Dyer. Used with permission.

This is what a total eclipse looks like. This is the total eclipse of October 27, 2004 via Fred Espenak of NASA, otherwise known as Mr. Eclipse. Visit Fred’s page here.